The present invention relates generally to wireless communication systems and, in particular, to channel assignment schemes.
Call quality and capacity are important concerns in wireless communication systems. Carrier signal to interference (C/I) ratio is a primary factor in determining call quality and capacity. Specifically, the higher the C/I ratio, the better the call quality and the higher the capacity. By contrast, the lower the C/I ratio, the poorer the call quality while potentially adversely affecting capacity. Several schemes exist for improving the C/I ratio. Two such schemes involve dynamic channel assignment (DCA) techniques and fixed multi-beam intelligent antenna (FMBIA) systems.
The first scheme, i e., DCA, is a flexible channel allocation technique for dynamically assigning communication channels to mobile-stations based on interference level measurements. Generally, communication channels having low or lower associated interference level measurements are assigned to mobile-stations before communication channels having high or higher associated interference level measurements. This scheme improves the chances that communication channel assigned to mobile-stations will have a high or acceptable C/I ratio.
A DCA technique in accordance with the prior art utilizes long term interference level measurements and short term or near real-time interference level measurements to dynamically assign communication channels. The DCA technique includes a channel segregation procedure and a dynamic channel assignment procedure, wherein the dynamic channel assignment procedure is based on the results of the channel segregation procedure. The channel segregation procedure involves measuring interference levels for each communication channel not currently active (i.e., idle communication channels) in a given cell or sector. The interference levels are measured using two different processes referred to herein as a long term process and a short term process.
The long term process is used to create a xe2x80x9clong listxe2x80x9d, which is then used by the short term process to create a xe2x80x9cshort list.xe2x80x9d The long term process involves measuring interference levels for all idle communication channels over a long term (i.e., every few minutes to few hours). These interference level measurements (also referred to herein as xe2x80x9clong term interference level measurementsxe2x80x9d) are used to create the long list, which is a list of the idle communication channels ranked in ascending order from lowest to highest average long term interference level measurements. The top portion of the long list includes the idle communication channels with the lowest average long term interference level measurements, and is referred to herein as xe2x80x9ccandidate channelsxe2x80x9d for channel assignment.
The short term process involves measuring interference levels for the candidate channels over a short term or near real-time (i.e., every few seconds to few minutes). These interference level measurements (also referred to herein as xe2x80x9cshort term interference level measurementsxe2x80x9d) are used to create the short list, which is a list of the candidate channels ranked in ascending order from lowest to highest average short term interference level measurements. Since the short list is updated more frequently than the long list, the interference level measurements of the candidate channels in the short list are more up-to-date than the interference level measurements in the long list resulting in perhaps a different order for the candidate channels (compared to the long list). Accordingly, the short list is used by the dynamic channel assignment procedure to assign candidate channels to mobile-stations.
The dynamic channel assignment procedure involves using short term Cest/Ishort ratios to determine whether a candidate channel is acceptable for assignment, where Cest is an estimate of a carrier signal strength based on the mobile-station""s uplink signal transmitted during call setup and Ishort is the measured average short term interference level. Several algorithms may be used to assign candidate channels to a mobile-station. For example, the candidate channel at the top of the short list (i.e., candidate channel with the smallest average interference level) may be the first channel assigned to a mobile-station if it has an acceptable Cest/Ishort ratio, or any candidate channel with an acceptable Cest/Ishort ratio may be assigned to the mobile-station. Therefore, DCA improves the chances that idle communication channels assigned to mobile-stations will have a high or acceptable C/I ratio.
The second scheme for improving the C/I ratio, i.e., FMBIA systems, involves using multiple narrow beam coverage in order to reduce co-channel interference to active communication channels. FIG. 5 depicts a geographical area or cell 10 associated with base station 14 for providing wireless communication services to mobile-stations within cell 10, wherein base station 14 has incorporated a FMBIA system in accordance with the prior art. Cell 10 is divided into a plurality of 120xc2x0 sectors 12-j, wherein each sector is further divided into four 30xc2x0 sub-sectors.
FIG. 6 depicts a schematic of a generic fixed multi-beam intelligent antenna system 20 for a time division multiple access (TDMA) based wireless communication system in accordance with the prior art, wherein communication channels are defined by a frequency and a time slot. FMBIA system 20 comprises an antenna array 22, low noise amplifiers 24, RF switch matrix 26, radios 28 and intelligent antenna controller (IAC) 29 (implementable in a digital signal processor). Antenna array 22 produces twelve 30xc2x0 beams for providing wireless communication coverage to the sub-sectors of cell 10. The 30xc2x0 beams are channeled to radios 28 via low noise amplifiers 24 and RF switch matrix 26. Radios 28 are assigned to process (e.g., modulate and demodulate) signals on active communication channels, and are equipped with beam scan receivers for sequentially sampling and measuring received-signal strengths (RSS) of the twelve beams for each time slot in each frequency channel. Each RSS is mapped to a RSS indicator (RSSI) value using a RSS-RSSI conversion or look-up table, wherein higher RSSI indicates higher signal levels for active communication channels. The RSSI are provided to IAC 29, which uses the RSSI to select an optimal beam or beams (i.e., beam with highest RSSI moving average) for each active communication channel. Upon selecting the optimal beam or beams, IAC 29 directs RF switch matrix 26 to channel the optimal beam or beams to the appropriate radios 28 assigned to process signals for the corresponding active communication channels. Thus, FMBIA systems improve the C/I ratio on active communication channels at mobile-stations by directing optimal beams to the mobile-station.
The C/I ratio in FMBIA systems can be further improved through the use of dynamic channel assignment techniques adapted for FMBIA systems. Accordingly, there exists a need for a dynamic channel assignment scheme adapted for FMBIA systems.
The present invention is a method for improving call quality and capacity by integrating a dynamic channel allocation technique into intelligent antenna systems. Channels are dynamically assigned to a mobile-station by measuring interference levels on each beam in a set of beams for a plurality of communication channels, and assigning to the mobile-station a communication channel from the plurality of communication channels based on the measured interference levels on each beam in the set of beams.
In one embodiment, the present invention measures long term interference levels on each beam in a set of beams for a plurality of communication channels, measures short term interference levels on each beam in the set of beams for a portion of the plurality of communication channels, and assigns to a mobile-station a communication channel from the portion of the plurality of communication channels based on the measured short term interference levels on each beam in the set of beams.